Some parts of the configuration format have been changed and are no longer readable by the 1.3 series. This - in particular - includes: playlists, media library state, File Operations preset list. If you have a backup of your pre-1.4 configuration, restore it if going back to an older version.

In general, reverting to an older foobar2000 version is not recommended.

All components compiled for version 1.3 should be compatible with version 1.4. Some components - decoders in particular - can be recompiled with the new SDK to utilize the new decoder priority preferences page.

Third party components may support the new functionality while remaining compatible with 1.3 series.

However, components specifically compiled for version 1.4 will not load in 1.3 series.

A new more efficient format is used for storing playlists and media library data, to rewrite as little as possible on each shutdown, contrary to rewriting everything on every shutdown which previous foobar2000 versions did. Unfortunately the new format is not compatible with older foobar2000 versions.
Your old playlists are retained in your foobar2000 profile folder; a new folder ("playlists-v1.4") is created for playlists saved in the new format. You can manually delete the old folder if you no longer need it.

File Operations presets are now stored in a plain text file which can be freely edited by hand.